Hygrometer



April 10, 1951 Q Q M|N1ER 2,548,550

HYGROMETER Filed 0012. 4, 1944 3 Sheets-Sheet 1 B 20 455 /0 a m ze .sa 4a AMM wm TMPE/effrvef f/v "C ATTORNEY April l0, 95l Qq; M|NTE`R 2,548,550

HYGROMETER Filed`\0ct. .4, 1944 3 Sheets-Sheet 2 INVENTOR JZ' cime/f5 C. JVM/727.

WD7 Mw Wm ATTORNEY C. C. MINTER April 10, 1951 HYGROMETER 3 Sheets-Sheet 3 Filed Oct. 4, 1944 INVENTOR CZ/F/E C M//y'F/e. 5f

BY m WMM ATTORNEY Patented Apr. 10, i951 UN ITED STATES PATENT OFFICE v2,548,550Y t HYGROMETER Y Clarke C. Minter, East Orange, N. J. vApplication October '4, 1944,V Yi'Serial No. 557,130

9 Claims.

The '-present invention relates to measuring apparatus `and more particularly `to hygr'om'eters operating :on Jithe psychrometric principle to give a idirect reading -of the relative v'humidity :regardless 'of -the temperature 1- of the atmosphere..

Hygrometersof several types are known-to the art but 'the -onlyf'type heretofore --k-nown 'for giving a direct read-ing Ais-the Jhair-hygrometer which is very-fragile and unreliable. `-Another `type equally well known is the wet and dry bulb `hygrometer which much more accurate than the hairhygrometer but which is subject to--the disadvantage that it is not direct reading. Consequently humidity reading -has to be determined from empirical tables or charts based on temperatures of the-wet bulb andthe-drybulb with the difference therebetween empirically establishing psychrometric tables vcorresponding to vreadingsof relative humidity.

Attempts have `:been made to produce a direct reading hygrometer operating on the psychrometric principle utilizing the temperature differencebetween a kwet'bulb anda dry bulb, but these have Vproven 'unsatisfactory vand unreliable because they fail toA take into account and compensate for certainbasic principles. For example, extensive experiments with the .wet and `dry bulb hygrometer have shown that the diierence 'in temperature Vlbetween the wet bulb andthe dry bulb varies from zero at .100% relative humidity to a maximum at zero "humidity `and that this maximum dilerence at zero humidity, "or any other humidity, increases 'as 'the 'tem'peraturejof the air, indicated 'by 'the dry bulb, increases. However', an analysis of the ipsyc'hrometric tables fails to reveal any obvious workable re'lationb'etween the lowering ofthetemperature of the wet bulb at a given humidity and the temperature of the dry bulb inorder that a direct-,reading hygrometer canreadily be designed totakeinto account such relation.

Appreciating the diiiculty a direct-reading hygrometer operable on thepsychrometr'ic principle has been suggested wherein the relation "between the dry bulb temperature, the relative humidity, and the psychrometric difference in temperature between wet and dry -bulb is expressed by a quadratic equation. .I have found, however, -that such a quadratic :relation is .not necessary because therpsychrometric difference in temperature increasesmore or .less linearly with temperature, except at very low humidity and very vhigh*temperature, conditions very 4rare in the inhabitedtareas of the world.

.It isaccordingly-an object-of v'the present invention-jtoprovide a y direct-reading Vhygrometer op erable on the psychrometric -principle wherein the accuracy between a wet and dry-bulb isrutilized.

Another obj ect ofy the, ,presenti Iinvention -isfthe provision of a hygrometer YVfor directly reading the concentration of flammable or poisonous vapors surrounded by a iilm of a volatile liquid other than water.

Another object of the present v:invention is --to provide a hygrometeroper-able :on the msychrometniciprinciple whereinfquantitative mechanical forces or displacements are employed and which are @produced an proportion to :concentrations of vapor of a volatile liquid in a body ofigas with such forces directly operating .v-avratio .meter Ito give an indication of humiditybr vapor fconcentration.

Still further objects of the .present ,invention will become obvious to those skilled in the art by referenceto the accompanying drawings wherein:

`Figure "1 'is Va1'graphic Aillustration of how 'the psychrometric d-iierence between the tem-perature of a wet anddryi'bulb for a given-humidity varies with changes `rin ambient temperature; vFigure '-2 isa graphic illustration showing -how the psych-rometric Jdifference Cbetween "the temperature cfa wet-'bulb and a-drybulb varies with humidity at a fgiven temperature;

Figure "3 is 'a lgraphic illustration showing the slight percentagegof error 'existent in employing a direct-"readingindicator with alinear scale Figure 4 is a 'graphic-illustration showing-'the relationship 'be'tween 'the Yvarious `forces utilized inthe -hygrometer `o'fy the presen-t invention lat various temperatures;

`-Figuref5 is Yaggraphic "illustra-tion `showing "the relationship between-the mixture in the wet rand dry Vbulb -at 4various temperatures and ythe vapor pressurev of 'the xmixtmres Aat such temperatures;

Figure 6 is la 'schematicjdrawing of I'the electrical arrangement'of fa-hygrometer constructed accordance with-the present invention land show-- ing'the novel construction -offa `ratio meter em ployed therewith;

Figure "'gis 'a schematic drawing of a modication whichifthe lelectrical v)circuit may 'take inthe hygrometerof the present invention;

' -Figure -8 is anelevational view of Aa mechanical form which the hygrometer of the 'present :invention may take, and

Figure 9 is an elevational view 'offra modication which the mechanical form of the hygrometer of the present invention VTmay Aft/jake.

Before describing in 4detail the various modiiications of the hygrometer of the presentfinventionvas `shown in thedrawingsit is lbelieved desirablehji'n.v aganmentionthat the diierence in temperature 'between 'the wet bulb and dryjbulb varies from zero at relative .humidityzo a maximum at zero humidity `and Ithat ,'t'hfsfmaximum diierence at zero humidity increases nas 3 the temperature of the dry bulb increases. It is this relationship which heretofore has constituted the greatest problem and so far as I am aware has not been properly considered in the production of an accurate direct reading hygrometer.

In my solution of the problem and because of the interpolations and extrapolations necessary in constructing a usable psychrometric table on which to base the design of an accurate directreading hygrometer, I find it expedient to employ a theoretical relation to calculate the difference in temperature between the wet bulb and the dry bulb as the relative humidity and the ambient temperature are varied. YThe relation I employ (A) Td-Tw=A :MPM-HPM) in which:

As will be hereinafter pointed out Equation A yields much more accurate results than are possible with any quadratic equation heretofore utilized and such results are much more useful than an experimental psychrometric table.

Referring now particularly to Fig. 1, wherein the abscissa represents the temperature of the dry bulb Td in C. and the ordinate the psychrometric difference or A, it will be readily seen how A for a given humidity varies with temperature, and in Fig. 2 how A varies with humidity for a given temperature. Yet it is these values of A which must be employed to operate a directreading hygrometer, and since the magnitude of A for a given humidity increases with temperature, as shown in Fig. l, it would clearly be impossible to obtain an accurate indication of the humidity by means of a xed scale and a A-operated pointer, without providing some means for automatically compensating for the yeffect of temperature on A for a given humidity.

By reference now to Fig. 2 wherein the ordinate again represents A and the abscissa relative humidity H, it will be noted that various curves have been drawn wherein the dashed lines depict the relationship between vthe lowering of the temperature of the wet and dry bulb and the relative humidity from to 40 C. These dashed lines have been calculated by means of an empirical relation:

(B) AFKQW) Au=psychrometric diierence at zero humidity An=psychrometric difference at any other humidity 'IT-:temperature H=relative humidity If it be assumed that this equation expresses the relationship between lowering of wet and dry bulb temperature and relative humidity, it will be clear that A is directly proportional to the temperature T and to (l-I-I). This assumption is not strictly true as shown by the full lines of Fig. 2 taken from actual readings but it can be readily seen that-the deviations from the theoretical As shown in dashed lines and actual readings as shown in full lines, is not excessively great except for very low humidities at rather high temperatures, conditions rarely encountered.

In Fig. 3, wherein the abscissa represents angular deflection in degrees and the ordinate percentage of relative humidity, the line A represents an angle of 45 which is the perfect correlation whereas the full line curve B depicts the error resulting by the employment of a direct-reading hygrometer with a linear scale as estimated from the graph of Fig. 2. The dotted line curve C of Fig. 3 shows a compromise calibration which gives greater accuracy over a wider range of temperatures and humidities though the error at low temperatures (approximately 0 C.) would be slightly increased.

From the foregoing the advantage in employing the relation is believed to be obvious, since for a given humidity A appears to be directly proportional to temperature and yet for zero humidity In order to employ a meter with a direct reading on a linear scale it therefore follows that the scale reading for a given humidity is the ratio That is to say, that for humidity the deflection of the meter or indicator must necessarily be zero While for zero humidity the deflection is a maximum. Accordingly in an accurate direct-reading hygrometer it is essential that the following requisites exist:

(l) A controlling or restraining force on the pointer of the indicator increasing linearly with temperature which follows from the formula which compensates for the effect of temperature on the psychrometric difference for a given humidity and which will be hereinafter referred to as controlling force.

(2) A deilecting force proportional to A K(T+ B) when the wet bulb is at the same temperature as the dry bulb with forces (1) and (2) equal at all temperatures for 100% humidity so that the meter deflection is zero.

(3) A deflecting force proportional to A superimposed on the deflecting force of (2) and acting in the same direction as the latter which may be termed a psychrometric force added to the deecting force (2) and in opposition to the controlling force that gives the reading on a linear scale of the relative humidity, with the sum of forces (2) and (3) being equal to force (1) at equilibrium.

The relationship of these forces may be better m agtirtt appreciated from Eig. f4'whereinthe Aabscissarep-V resents temperature in degrees C. and Lthe ordinate the relative forces for variousfhumidities. Considering rst the ,line D, this shows the relative magnitude of the deecting force (2) and the controlling force (1) on the indicator for various temperatures from zero to 40 C. at 110.01%v

ing the data as graphically represented in Figs.

Relative Humidity, H.

1.40-y r25.3 .21.4 17. 94 15. 00 12.15 0.05 7,45 5.35 3. 45 1. 75 0 ,35 22.3 19.0 15.1 13.451105 3.35 .0.32 4.95 3.151.53 o 30 19.4- 15. 75 14. 25 12.00 9. 90 i0.00 v5. 15 4.50 2901.40 0 25 15.05 14.50 12.55 10. 00 8. 70 7.10 5.50 4.05 2. 55 1.23 `0- 20 14.10 12.35 10.70 9.15 V7.55 5.20 4.87 3.50 2321.13 0 15 y11.-70 10.35 9.00 7.175 0. 50 5.30 4.15 3.07 2.00 0. 99 0 11o" 0.55 3.45 7.42 5.40 5.38 4.40 3.55 2.54 `1.53 0. 025 l0 5 7.50 0.75 5-94 5.13 4.41` 3.53 2.33 2.15 1410.70 o 5.73 5.10 4.50 3.00 3.34 v`2.75 2.15 I1.52 1.1'0 0.55. o

Having described the .requisites necessary to provide an accurate direct reading hygrometer operating-on the psychrometric principle, the `remaining figures of the drawings `show several embodiments of the present invention in which the necessary forces are .produced .either -electrically or mechanically.

Referring now particularly to Fig. .6 a Vwet bulb resistance thermometer Rw 4is shown having a suitable coeicient-of Iresistanceand forming one leg of a Wheatstone bridge whileRais-.a dry bulb resistance thermometer vhaving a higher coe'flicient of resistance than Rw. Connectedfin a second Wheatstone bridge arrangement with dry bulb Rd .are resistances 5,. S .and l1 which are not responsive to temperature .but all Vof which constitute a leg of the primary bridge, the latter including the wet bulbresistance Rw'as well as two inert resistances '8 .and v9. l

The junctionof resistances Band LSis connected toone side .of a voltage source, such as .a battery I0, while the junction .of wet bulb resistance Rw and the secondary bridge including the dry bulb resistance Rd, is connected to the other side of the .source of voltage I0. .Al pair of conductors 'I2 and I3 connect the opposite points of .theprimary bridge to Va winding I4 through torgueless springs (not shown) of a meter I5 'while .another pair of conductors y|56 and I 'I connect the remaining points of the vsecondary'bridge to a further coil I8 of the meter I5. j

As shown in Fig. 6 the metercoils I4 and "I8 are secured to' the same'shaft I9 but'at l90"""with respect to each other and apointer is carried by the shaft I9 with the' pointer registering the percentage of humidity on a scale22. The meter coil -or winding I4 moves -in a 'gap "23 between magnetic pole pieces 2l4randl25 in which the 'flux density is uniform-due to afuniform .spacing between the' pole pieces 24- andz25 throughouttheir 'adjacent arcuate :surfacesthrough which fthe vcoil I4 moves. 'On fthe bother hand the coil 'I8 `moves ina .gap 26 in which .the '.lux density :increases `in :alclockwisedirection inasmuchfas 'the spacing between adjacent arcuate 'Surfaces' Iof the po'ler' pieces 24 and 25 varies through which :coil '-118 moves. l Y

In the operation othe apparatus of Fig. 6 the primary bridge is unbalanced at all temperatures due .to the temperature .coei'cient of' resistancey of .the secondary .bridge .being greater than that of resistance Rw. This condition holds :trueieven Y when the temperature-of Lthe wet bulbis the :same

as `:that of .the vdry :bulb (100% humidity and they :have the .same :resistance 'at 'only 'one Vtemr perature, namely, Ythat corresponding tothe hypcthetical temperature calculated from the r fundamental .relation `.given above. Since it Vfollows Vthat which latter is the hypothetical temperature yat which the bridge is balanced.

Hence the primary bridge is unbalanced lat Iall temperatures, except --A/B, with the result that afcurrent flows through-coil I4 at all times'even when Rw and R51-are the same temperature. .This produces a torque proportional vto (A+BT) which ltends todeflect-the pointer 2.0 in aclockwise direction. The secondary bridge is also unbalanced at ,all times causing .a greater current to `flow through winding I8 .and producing a torquetending -to move the pointer 20 in a counter-clockwise direction .in .opposition to the torque produced by the winding I4. .The current through the winding VI8 follows the-same -rule of proportionality to .(A-l-BT) vand the values .of the wet and dry bulb resistances and their respective temperature coemcients are so chosen as to lproduce such proportionalityand-when at the same temperature to cause the pointer 20 vto .rest at 100% relative humidity.

This results in Ithefcoil I8 4being in thevweakest partfof the magnetic .field in the gap 265but when the temperature lof RW becomes less than-that .of Rayas when the vhumidity falls below V1,00%, the current through coil I4 increases `thereby developing Amore torque withclockwise rotation of the'pointer as well as :that of coils I4 and I8. Such movement, however, causes the coil I8, :in which the currentA has :remained constant, to foccupy a position in -themagnetic viield rof gap--26 of ,such .increased iiux density that .the .torque developed by winding I-Bequals ,the torque developed by :coil .I.4, .thus bringing -the pointer `2li to rest on .the-scale -22in registration with indicia indicatingthe relative humidity.

.It-can thus be seen .that vfor .any equilibrium position of the pointer 20the Ytorque Adeveloped by the two coils is equal and opposite even though the current vowingthrough the coils I4 and3I8 is never .the same except .at .the point-at the end of kthe :scale .corresponding to zero :hu-v midity. Accordingly at any .position of the pointer the torquesfdeveloped .in the two coils I4 and] 8 wouldfbe dependenton therespective currents which are calculated Vfrom the .above mentioned formula. .For example, the current for winding I4 would Abe .calculated '.by the torque relation:

whilerthe'f-'current fori-winding llgmeverbeing'fthe same as that for winding I4, would be proporin which F is the flux density in gap 23 and F is the iiux density in gap 26 for 100% humidity position of coil I8.

The modiiication of Fig. 'I diers slightly from that of Fig. 6 in that the secondary Wheat-stone bridge arrangement has been dispensed with as well as one of the meter windings. In lieu of such winding a bimetallic hair-spring 30 is utilized. Again, however, the juncture of the fixed resistances 8 and 9 is connected to one side of the source of constant voltage In while the juncture between the wet bulb resistance Rw and the dry bulb resistance Rd, which constitutes the other two legs of the Wheatstone bridge, is connected to the other side of the constant Voltage source I0. The remaining points of the bridge are connected by conductors I2 and i3, respectively, to one end of the torqueless hair spring 30 and to one end of a meter winding or coil 32 carried by a shaft 33 thus connecting the spring 3B and coil 32 in electrical series relationship with the bridge arrangement. Also connected to the shaft 33 is a pointer 34 which registers with a scale 35 in the same manner as in the meter of Fig. 6.

As in Fig. 6, the resistances 8 and 9 are similar and xed in that they have no coeicient of resistance, while the wet bulb resistance Rw has a temperature coefficient of resistance a and the dry bulb has a suitably higher temperature coefficient of resistance The torqueless hairspring 30, which is anchored at the end to which the conductor I2 is secured, controls the movement of the coil 32 between the pole pieces 36 and 31 of the magnet and opposes the torque developed in the coil 32 when current flows through the latter due to the difference in resistance between Rw and Rs.

Since the temperature coefcient of resistance of the dry bulb Rd, is greater than that of the wet bulb Rw, the bridge is unbalanced at all temperatures (even at" 100% humidity) in the same manner as previously described relative to Fig. 6. As the ambient temperature increases the dry bulb Rd increases in resistance faster than that of the wet bulb Rw thus increasing the unbalanced condition of the bridge with more current flow through the coil 32. The torque accordingly developed in the coil likewise increases tending to move the pointer 34 more and more to the right as shown in Fig. '7. However, the opposing torque of the hair-spring 30 likewise increases at the same rate as the ambient temperature increases, with the result that the pointer 34 remains stationary at 100% humidity, as shown by line D in Fig. 4. When the humidity is less than 100% force (3) becomes operating and more torque is developed in the coil 32. This torque acting in a clockwise direction winds up the bimetal hair-spring 36 until the torque thereof equals the torque of coil 32. The two opposing torques are then in equilibrium and the position of the pointer indicates the relative humidity on the scale 35.

The relationship of the resistanoes Rw and Rd, as well as the torque of the bimetallic hairspring, may be better appreciated when it is considered that at a temperature corresponding to T=-A/B both the wet bulb and the dry bulb have the same resistance R, in the same manner as. described relative to the current owing through the windings I4 and I8 of Fig. 6 for thel hypothetical temperature, but at any other temperature For any condition of temperature or humidity the torque developed in the coil 32, which is proportional to current, is proportional to: Y

while the torque of bimetallic hair-spring 3D is given by the relation In Figs. 6 and 'l I have shown the hygrometer of the present invention as being electrically operated since the requisite forces are created by the ilow of current through two separate coils as in Fig. 6 or the employment of current flow through a coil to create a force in opposition to that of a bimetallic hair-spring as in Fig. '7. Since, as previously mentioned, the hygrometer of the present invention depends for its operation on forces which are proportional and opposite such forces may be readily produced mechanically as well as electrically.

By reference now more particularly to Fig. 8 one mechanical form of a direct-reading hygrometer of the present invention is shown comprising a housing 4I] encasing an upper eXpansible bellows 42 and a lower similar bellows 4t having their opposite ends interconnected by a shaft 44 while their remaining ends bear against opposite walls of the housing 40. 'I'he upper bellows 42 contains a volatile liquid 45 and its vapor 46 which exerts a pressure tending to push the lower plate 4'I in a downward direction.

The lower bellows 43 likewise contains a vapor 48 of a volatile liquid 49 contained in a tube 50 which communicates with the interior of the bellows 43, the volume of which is decreased by a solid material 5I, so that the vapor 48 therein exerts a pressure tending topush the bellows plate 52 in an upward direction in opposition to the force exerted lby upper bellows plate 41 through shaft 44. A lever arm 53 is pivotally connected at 54 to the shaft 44 and such lever arm is arranged to rotate about a pivot 55 carried by a housing bracket 56. The remaining end of the lever arm 53 is loosely linked at 5l to a second lever arm 58 pivoted to a shaft 59 and carrying a quadrant 60 while the free end of the second lever arm 58 is connected to a bimetallic spiral 62 in turn pivoted at 63.

The relative forces shown in Fig. 4 are all based on the assumption that the forces increase with temperature according to the relation It is accordingly obvious that the volatile liquid 45 in the bellows 42 cannot be a pure liquid since the vapor pressure of a pure liquid would vary in an unsuitable manner as shown by the upper curve of Fig. 5. In order that the vapor pressure of the liquid 45 in the bellows 42 increase with temperature in a linear manner, as shown by the mixture of dry bulb curve of Fig. 5, it is necwhile Vessary 'to 'use a mixture .consisting -of a volatile liquid dissolved in anon-volatile liquid and the volume of the liquid lmixture must be .small compared with the volume-.of the -vapor space.

If these conditions are fullled it follows that tion of the Volatile component and the total volume of the mixture, the desired linear vapor pressure curves for the liquids in the wet and dry bulbs can be obtained as shown in Fig-5.

It is also necessary that the two liquids `be miscible and that they `obey Raoults law; conditions which vcan be met by selecting two components which are chemically related. As an example, if ootyl-chloride, which boils at 1831C., is taken as the non-volatile component and methyl-chloride (C1-1301) rwhichlboils at r24? C., is selected as the volatile component, then the linear variations in vapor pressure can be -obtained for the two components as required to give the wet and dry bulb cur-ves of Fig. 5.

Still further mixtures meeting the above conditions and comprising a lmixture of a nonvolatile liquid and a highly 4volatile liquid, are some of the hydrocarbonsrsuchas ethane boiling at 88 C. and Adecane boiling :at :1.74" C. which can be employed. .It .might valso be .mentioned that if the vapors .of .the volatilecomponentfdo not obeythe simple gas laws, i. e.,expandlinearly vwithincreasing temperatureno .appreciable error Vis introduced since thesame vapor is in both the wet and dry bulbs or bellows 42 .and43 Yand .itis only the difference in .pressure of the two vapors with which one-is here concerned.

The manner in which the exact quantities of both the Volatile component .and .non-volatile component requiredin the .liquidmixtures 45 and v#'19, to obtain the Vlinear pressure-temperature curves as shown in Fig. .5, are properly calculated may be better understood from. the following:

According to Raoults law, the vapor pressure of the volatile component of such a mixture is proportional to the mole fraction of the volatile component in the mixture multiplied. by the vapor pressure of the volatile component in the pure state. That is:

wherePt is'the vapor-pressure of the-pure volatile .component at Y the temperature .T, vand .the mixture contains 'l mole .of the `non-volatile com- .ponent :and .n lmoles .of the -volatilecomponent.

centration of the-volatile component in the liquid,

with Vthe .result Athat .the vapor -pressure of the volatile .component .the mixture decreases .as vaporization goes to equilibrium.

It .can .thus be `seen .that with. a :small volume .of Vthesolution in the bellows vl2 .and .43, .the concentrations or mole Vfractions (n/ (n4-1)) will decrease as the temperatureincreases, with .the net result that the vaporfpressure of the volatile l.component in the mixture will increase more slowly with temperature than if the volatile component were in the pure state, or if the volume of the vapor space were small compared to the volume of the mixture. 45 in the upper bellows 42 of Fig. 8 and assume it is desired to have. va linear variation of vapor pressure in the bellows 42 as the temperature is increased, and the vapor pressure follows the re- Equation 2 gives the vapor pressure of the liquid phase and according to the gas laws the pressure in the vapor phase must be:

in which ng is the number of moles of vapor in the vapor phase contained inthe volume V, and K is a constant.

At equilibrium (2) and l(3) must be equal, or

Solving for ng we obtain as the number of moles in the vapor phase Equation 5 givesthe number .of moles ng the vapor phase at lany temperature. At 16.5 C. (see Fig. 5) thenumbenof moles in the vapor phase would be 5v .1+(16;5+'T "i 7) K WT .0039] l The moles Vin solution at any temperature Would be nofminus (7), and the mole fraction would be Combining (l), (2) and (8'), we have, Aafter reducing and solving for no, i n

PDV Pd Pd nf" K 273+T Y0039)+P.-PdA

Pt and Pd can be taken .direct from the graph of Fig. 5.

.If the Psin (9) are expressed in atmosphere, then K must vbe in liter-atm. r(.0820'5). The volume of the vapor space V .will vthen-haveto be expressed inliters in order to obtain no in moles. For a given value of Po, only one `particular value of V .will give .the .same value for mas Pt andPd .are varied. vmand V are there- .'foreclosely linked with the equation for Pd. Also Taking rst the liquid Vmixture 45 suitable for the bellows 42, it is believed obvious that they apply equally to the wet bulb or liquid 49 for the bellows 43 of Fig. 8, and the vapor pressure of the liquid will vary with temperature as shown by the lower curve of Fig. 5.

Having computed the liquid and vapor pressures in the above manner, the liquid 45 in bellows 42 is given a higher vapor pressure than that of the liquid 49 communicating with the lower bellows 43, as shown in Fig. so that when these two liquids 45 and 49 are at the same temperature (as would be the case for 100% humidity) the vapor 46 in bellows 42 exerts a greater pressure on end plate 41 than the vapor 48 exerts on end plate 52 of the lower bellows 43 which would cause downward movement of pivot 54 with counter-clockwise rotation of lever arm 53 and attendant clockwise rotation of lever arm 58, if

the latter were not restrained by an equal force exerted upward at the other end of lever arm 58 by the bimetallic spiral B2.

The force thus exerted by the bimetallic spiral 62 is the controlling force which varies with the temperature according to the formula Kurt) While the force due to the difference in vapor pressure in the bellows 42 and in the bellows 43` so that at 100% humidity the force exerted by the difference between the vapor pressures of 4B and 48 is equal to that exerted by the bimetallic spiral 62, as shown by the curve D in Fig. 4. Again referring to Fig. 8 it will be noted that the volatile liquid 49 is surrounded by a wick or the like 84 which is wetted by a liquid such as distilled Water 85 contained in a reservoir 66.

When the relative humidity of the atmosphere surrounding the wick 84 is less than 100% the wick being wetted by the distilled water is at a lower temperature than the surrounding air. This accordingly lowers the temperature of the volatile liquid 49 within the tube 50 which is surrounded by the Wick B4 causing the pressure of the vapor 48 within the bellows 43 to decrease with attendant decrease in the upward pressure on the bellows end plate 52. The force exerted by the-vapor pressure 46 within bellows 42 thus predominates and causes downward movement of pivot 54 together with counter-clockwise rotation of lever arm 53 about its pivot 55 and attendant clockwise rotation of lever arm 58 about the shaft 59 and against the opposing force exerted by the bimetallic spiral 62 until the latter increases in tension and becomes equal to that exerted by the difference in vapor pressure within the bellows 42 and 43.

Since the quadrant 80 is provided with a toothed rack 61 meshing with a gear 68 rotatable about a shaft 69 and the gear carries a pointer 10, the latter moves to the left, as shown in Fig. 8, to a new position on a scale 12 calibrated in terms of relative humidity. Thus it will again be seen that the apparatus as shown in Fig. 8 employs the same three forces as previously described in connection with the preceding figures which forces are produced mechanically instead of electrically but they nevertheless follow the formula above mentioned.

When the humidity is less than 100% any movement of the bellows in Fig. 8 will decrease the volume of the vapor space in the bellows 43 and increase the vapor space in the bellows 42. This will cause an increase in the concentration of the volatile component in the mixture 49, and a decrease in the concentration of the volatile component in the mixture 45. This would tend to decrease the pressure difference and introduce an error, which is not large in any case, but which can be kept to a minimum by having the total volume of the vapor space as large as possible and the diameter of the bellows as small as possible.

Since any movement of the bellows 42 and 43 requires a certain force to overcome the natural spring action of the bellows, which is independent of temperature, as well as the spring action of the bimetallic spiral 62 which increases with temperature, it is obvious that at low temperatures the spring action of the bellows 42 and 43 for a given movement would form a higher percentage of the total spring action, and an error would result from this cause. However, this error can be eliminated by setting the bimetallic spiral 62 to start its action at some temperature higher than -A T- B the exact point depending on the magnitude of the spring action of the bellows 42 land 43.

The modiiication of Fig. 9 in principle is similar to that of Fig. y8 but the various above mentioned forces are produced by diaphragms in lieu of expansible bellows. For example, a diaphragm 15 is formed of an evacuation tube 18 and a wet bulb 11 containing a volatile liquid 18 and rigidly held in place by an adjustable set screw 18 passing through a post attached to the casing (not shown) for the apparatus. A similar diaphragm 82 is also formed with an evacuation tube 83 and a dry bulb 84 provided with a volatile liquid 85, being also rigidly held in place by an adjustable set screw 86 attached to a post 81 forming part of the casing, inthe same manner as the post 80.

A rigid bar 88 positioned between the two diaphragms 15 and 82 keeps the spacing therebetween constant and such spacing is regulated by theadjustable set screws 19 and 86 in the respective posts 80 and 81.

A pin 89 is carried by the rod or bar 88 which engages a bifurcated portion of a sector 90 pivoted to a shaft 92. This sector engages a pinion gear 93 carrying a pointer 94 registering with a scale 95, in the same manner as above described relative to Fig. 8. A second pin 96 is also carried by the rod 88 which is contacted by a strip 91', of heat responsive material such as bimetal, adjustably secured by set screws 98 to a supporting block 99, the latter of which is adjustably secured as by a set screw |00 to a post l02 forming part of the casing (not shown).

An increase in temperature tends to cause the bimetallic strip 91 to deflect from left to right as shown in Fig. 9, but it is restrained by the pin 96 carried by bar 88. rConsequently as the temperature increases and when both the wet bulb 11 and dry bulb 84 are at the same temperature (100% relative humidity), the vapor pressure in diaphragm 82 must be greater than the vapor pressure in diaphragm 15 by an amount equal to the force exerted on the rod 88 by the bimetallic strip 91. It therefore follows that the vapor pressure of the liquid 85 in bulb 84 must be greater than that 0I the liquid 18 in bulb 11 and that the vaporpressuress will be the. same, at one; tem.-A

perature, namely.. Y

Ting

Thisl means that the difference: will increase..r at the same rate,. when the liquidsv areatrthe. same temperature for 100% relative humidity, asi the restraining pressure. exerted' by theVV bimetallic strip. 9.1' on bar 88 resulting in thebar'remaining stationary even with changes: inV temperature. at 100% relative humidity.V However,.whenthe;hu. midity'becomes less thanz100%= the liquidi l8i will be ata lower temperature than thatlof theliquid 85' and the diierenceinvapor pressure therebetween willi increase..

In the same mannerY as abovel described relative. to= Fig. 8', the opposing force of bimetallic strip 91. then becomeslless-than` thediierence` in vapor: pressure between the diaphragms and 82. with the result thatthe bar 88 will thenlmovev from right to left; as viewed in Fig; 91; againstthe spring action exerted by the birnet'allic4 strip.

As; soon as the spring action oft the bimetallic strip 01. isV suilicientlygreat-to balance the-y force on thev bar 88' due: to. the diirerenee in vapor pressure in diaphragms 'I5 and 82 equilibrium will. be reached and. theY pointer 94 which rotates upon movement of bar 88 will indicate on the scale 95 the relative humidity:

Fronr the. foregoing; it willf. become` obvious. to those skilled in the art that a1 direct-reading hygrometer hasY been herein provided which is operable on the psychrometric principle. Moreover; thenecessity for charts and tables has-been eliminated' by the` constructionof a hygrometer employing varying forces mathematically calculated. sol as toV give the correctproportion under all. conditions from zero=t`o1-100f% relative=humidi ty which results in an accurate meter indication under all conditions. These forces can be produced either electrically or mechanically and are responsive to temperature changes so that by utilization of a ratio meter they are converted in terms of relative humidity on the meter scale.

Although several embodiments of the present invention have been shown and described, it is to be understood that other modifications thereof may be made without departing from the spirit and scope of the appended claims.

I claim:

1. A direct-reading psyohrometric hygrometer comprising a meter for registering the relative humidity, means operable to subject the movement of said meter to a deflecting force tending to cause the movement of said meter to move in one direction including a dry temperature responsive element and a wet temperature responsive element having a lower temperature coefficient of response, and a dry temperature responsive means operable to subject said meter movement to a deflecting force in an opposite direction.

2'. A direct-reading psychrometric hygrometer comprising a meter for registering the relative wherein 'I'. equalsa temperature. in. degrees' centigrade.; A equals: 5.8,l and; B: equals; 0.345; and a dry temperature responsive means operable to subject said metermovement to an oppositely directed. force increasing with temperature. according to the same. relation 3; A direct-reading.A psychrometric hygrometer compris-ing a` meter for registeringV the. relative humidity, means operable to subject the movement-of'said meter. to a deflecting forcey tending to cause the movement of said: meter to move in one= directionv including a dry temperature responsive element. anda wet temperature. responsive element having a.. suitably lower temperature coefficient of response suchthatwhen the relativehumidityis saidforceincreases with.- temperature according to the relation wherein T equals temperature in degrees centigrade, A equals, 5.8,, and B equals 0.345; andv a dry temperature. responsive mea-ns.' operableY to subject said meter movement to an oppositely directedforce increasing with temperature: according to the same relation 4.. A direct-reading psychrometric hygrometer comprising means for producingfa:directedforce including a. dry temperature. responsive element, means for producing an oppositely directed force including a dry temperature responsive element and a wet temperature responsive element having a suitably lower temperature vcoeflcient of response such that when the relative humidity is 100% the said oppositely directed force increases with temperature at the same rate as said rst mentioned force, means for bringing the two said forces in opposition to each other, and means for indicating when the two said forces are in equilibrium including a movable pointer and a dial having a scale calibrated from 0% to 100% relative humidity.

5. A direct-reading psychrometric hygrometer comprising a meter subject to a plurality of forces for registering relative humidity, electrical means operable to subject the movement of said meter to a deecting force tending to cause the movement of said meter to move in one direction including a dry temperature responsive element and a wet temperature responsive element having a lower temperature coefficient of response such that said deecting force actually applied to said meter movement varies linearly with temperature and is proportional to the psychrometric difference-between the temperature of the dry" and wet temperature responsive elements, and electrical means including said dry temperature responsive element operable to subject said meter movement to a deflecting force in the opposite direction until such opposing forces reach equilibrium and said meter registers the relative humidity.

6. A direct-reading psychrometric hygrometer comprising a meter subject to a, plurality of forces for registering relative humidity, electrical means operable to subject the movement of said meter to a deiiecting force tending to cause the movement of said meter to move in one direction including a dry temperature responsive element and a Wet temperature responsive element having a lower temperature coefficient of response such that said defiecting force actually applied to said meter movement varies linearly with temperature and is proportional to the psychrometric difference between the temperature of the dry and wet temperature responsive elements, and a dry temperature responsive means operable to subject said meter movement to a deflecting force in the opposite direction until such opposing forces reach equilibrium and said meter registers relative humidity.

7. A direct-reading psychrometric hygrometer comprising a meter subjected to a plurality of forces for registering relative humidity, means operable to subject the movement of said meter to a deflecting force tending to cause the movement of said meter to move in one direction including a pair of pressure elements comprising a Wet bulb and a dry" bulb having 9, difference in pressure therebetween which diierence varies With temperature according to the relation wherein T equals temperature in degrees centigrade, A equals 5.8, and B equals 0.345; and a member operable to Subj ect said meter movement to a deflecting force in the opposite direction until such opposing forces reach equilibrium and said meter registers relative humidity.

8. A direct-reading psychrometric hygrometer comprising a meter subjected to a plurality of forces for registering relative humidity, means operable to subject the movement of said meter to a deflecting force tending to cause the movement of-said meter to move in one direction including a pair of pressure elements responsive to a "dry bulb and a wet bulb having a diierence in pressure therebetween which diierence varies with temperature according to the relation wherein T equals temperature in degrees centigrade, A equals 5.8, and B equals 0.345; a pivoted member included in said meter movement and operable about its pivot in response to said deflecting force, and a temperature responsive member operable to subject said meter movement to a deflecting force in an opposite direction by causing rotation of said pivoted member until the resultant opposing forces reach equilibrium and said meter registers relative humidity.

9. A hygrometer comprising a meter subject to a plurality of forces provided with a pair of coils for registering the relative humidity, an electrical circuit including a, primary Wheatstone bridge provided with a wet temperature responsive element in one leg thereof and having a diagonal bridge connected to a coil of said meter tending to cause movement of said meter in one direction in response to variations of said element upon the flow of electrical energyin said circuit, a secondary Wheatstone bridge constituting one leg of said primary bridge, a dry temperature responsive element in one leg of said secondary bridge and a diagonal of said secondary bridge connected to another coil of said meter to cause movement of said meter in an opposite direction upon the flow-of electrical energy through said latter coil.

CLARKE C. MINTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS I Number Name Date 1,098,472 Bristol June 2, 1914 1,169,617 Comfort Jan. 25, 1916 1,956,386 Gruss Apr. 24, 1934 `1,984,341 Grebe et al Dec. 1l, 1934 

